#include <bits/stdc++.h>

using namespace std;

long largestValue(vector <int> A,int n) {
    vector<int >b; 
    long sum=0;
    int i;
    // Return the largest value of any of A's nonempty subarrays.
    for(i=0;i<n;i=i+2)
    {
         sum+=A[i]*A[i+1];
           }
    return sum;
    
}

int main() {
    int n;
    cin >> n;
    vector<int> A(n);
    for(int A_i = 0; A_i < n; A_i++){
       cin >> A[A_i];
    }
    long result = largestValue(A,n);
    cout << result << endl;
    return 0;
}